1. Field of the Invention
The invention relates to a plain bearing shell with bearing backing and single- or multi-layer bearing material applied thereto.
2. Description of Related Art
In internal combustion engines, light-weight construction methods, i.e. the use of light connecting rods and bearing caps and the use of light metals, are preferable as they enable energy savings to be made. A consequence of this is that the bore for accommodating the plain bearing becomes widened and deformed under load, whereby relative movement occurs between the bearing backing and the receiving bore. In order to secure the bearing shell in the receiving bore so that it does not rotate, the external diameter of the bearing shell is designed to be larger than the internal diameter of the receiving bore. When the bearing is installed in the housing, tension arises as a result of the oversize of the bearing shell, said tension taking the form of compressive strain .sigma..sub.L in the tangential direction and radial pressure .rho..sub.r between the bearing and the housing, the latter determining an interference fit.
When the engine is running, however, the interference fit deteriorates as a result of deformation of the housing and creeping of the steel in the bearing shell as a consequence of the high temperature, whereby the radial pressure diminishes. According to "Die Berechnung des Pre.beta.sitzes von Gleitlagerschalen", Dr. E. Romer, offprint from MTZ, volume 22, issue 2, in the case of connecting-rod bearings the radial pressure should therefore amount on average to at least approximately 10 N/mm.sup.2 in order to ensure a reliable interference fit of the bearing during operation.
However, in the design of the bearing shell it is also necessary to take into account the compressive strain .sigma..sub.L, which is greater by approximately the factor 20 than the radial pressure .rho..sub.r and is thus responsible for the actual stressing of the bearing.
Since loading depends on the cross-sectional area of the bearing, it follows that the stress increases if the degree of oversize remains constant and wall thickness is diminishable.
For this reason, only bearing shells with a total wall thickness .gtoreq.1.4 mm (housing bore diameter of 50 mm, i.e. the ratio of the effective wall thickness to the external bearing diameter W.sub.eff /D is greater than 0.02) have hitherto been used. The effective wall thickness W.sub.eff is the sum of the thickness of the individual layers, which are standardised in such a way as to take into account the maximum modulus of elasticity of the multi-layer system.
From "Die Berechnung des Pre.beta.sitzes von Gleitlagerschalen", Dr. Erich Romer, offprint from MTZ, volume 22, issue 2 and 4/1961 and "Aspekte zur Gleitlagerung von Nutzfahrzeug-Dieselmotoren", Dr. Erich Romer, offprint from MTZ, 79th volume, No. 9/77, it is known that the ratio W.sub.eff /D is from 0.03 to 0.05 or, in the case of connecting-rod bearings, the ratio of the wall thickness W to D is from 0.02 to 0.03.
In the case of connecting-rod bearings, the residual radial pressure is of particular significance, since, in this instance, the deformation of the receiving bore is very great owing to varying tensile and compressive loads.
When an engine is running, extremely rapid load variation arises owing to the piston moving in the axial direction. This means, as is shown in FIG. 1, that a conventional bearing shell 2a, 2b cannot continue to follow the deformation movement of the bore 3 adequately and the bearing backing momentarily becomes detached from the housing 1.
This leads to two problems:
1. As a result of the momentarily arising gap 4, oil penetrates between bearing backing and receiving bore 3. If the gap closes again, a residual amount of oil is shut in, whereby in time oil carbon builds up on the bearing backing. PA1 2. As a result of the deformation, the bearing shell 2a, 2b no longer fully adjoins the housing 1, whereby the radial pressure is no longer able to act to its full extent. In order then to secure the bearing against rotation in the housing 1, the radial pressure must exhibit the above-described minimum value. PA1 good heat dissipation from the plain bearing surface, PA1 low connecting-rod bearing weight leads to a reduction in centrifugal forces in high-speed engines, PA1 very good workability during manufacture (especially in the case of mass-production), PA1 the plain bearing shell causes only slight deformation of the receiving bore through bearing oversize.
For this reason, in the prior art known hitherto, wall thicknesses were maintained which would ensure a minimum radial pressure, in order to reduce oil carbon build-up on the bearing backing. On the other hand, the bearing backing was machined in diverse ways, in order to divert the oil penetrating between bearing backing and receiving bore and thereby to reduce oil carbon build-up.
However, these bearings were extremely complex to manufacture and, moreover, were not particularly effective.
A plain bearing arrangement of this type with a thin-walled plain bearing element is known from DE 33 28 509 C1. To prevent the build-up of oil carbon between the backing surface and the receiving bore, fine channels are formed on the bearing backing to act as drainage channels for liquid lubricant. The drainage channels are intended to make it possible for the lubricant to escape towards the free ends of the bearing surface during relative movement. The pressure and temperature conditions required for oil carbon production are thereby avoided.
U.S. Pat. No. 2,905,511 describes bearing shells which in part have a total wall thickness of only 1.07 mm and in addition also comprise grooves or recesses, no indication being given as to external diameter, however.
The provision of drainage grooves or recesses makes the manufacturing process highly complex, however.
GB 256 200 discloses a similar attempt at solving the problem, according to which the bearing backing has projections which rest against the bearing bore. The bearing shell is altogether more flexible owing to the reductions in wall thickness between the projections.
Another solution is described in EP 0 304 109. To ensure the interference fit in the bearing housing under all internal combustion engine operating conditions, a 0.5 to 5 .mu.m thick metal protective layer is provided on the back of the steel backing layer.